You will experience a pH drop injecting co2. How much depends on the hardness of your water. KH= 4 or above and you'll be fine. The average is a 1 degree drop in pH, which is usually a good thing because most people's water is a little basic(7.2, 7.4) and most plants like slightly acidic water(6.4, 6.6).
As far as costs. Excel is the worst option, will cost you the most money in the long run. You can setup a DIY co2 system for under $20 using stuff lying around the house. Then you have to buy sugar. With a 20g, DIY co2 is fine for a couple of years. After a few years, the total costs exceed what the costs would have been to purchase a pressurized system.
On larger tanks, the sugar you use makes DIY co2 become more expensive than pressurized in under a year. I'm doing DIY co2 on a 45g. 30 ppm of co2 in the tank(the desired level) so no problems there, but sugar adds up.
On top of that, I have another 46g tank that I'll be planting soon, so now I'll have 2 tanks, doubling my sugar costs. Furthermore, my next tank(year or two down the road) will be over 100g. Therefore, I'll be going pressurized very soon.
Point is, put together a diy co2 system and set the excel aside. Again, check out the sticky about diy co2. If you ever go above a 30-40g tank, then it's time to save your pennies for the up front cost of a pressurized system.
